With the development of communication technologies, multiple types of services or traffic have been proposed, for example, enhanced mobile broadband (eMBB), massive machine type communication (mMTC) and ultra-reliable and low latency communication (URLLC). These services require different Quality of Service (QoS), for instance, delay, data rate, packet loss rate, and so on.
Generally, URLLC requires low delay and/or high reliability, but usually it also has very low data rate and possible sparse data transmission. mMTC typically requires long battery lifetime but does not require low delay or high data rate, often combined with small infrequent packets. With regard to eMBB, it generally requires high data rate. Delay for eMBB may be strict but typically less strict than in URLLC.
Conventional, retransmission functionalities are used in multiple layers to avoid: Automatic Repeat Request (ARQ) in Transmission Control Protocol (TCP), Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) and Hybrid Automatic Repeat Request (HARQ) in Medium Access Control (MAC) to avoid the packet loss in the whole transmission path including the transport network and Radio Access Network (RAN). For different services, there are different configurations of the mentioned layers according to the QoS requirement. For eMBB, the data is not so delay sensitive but the data rate should be as high as possible and the packet loss should be avoided. Hence the data retransmission of all layers shall be applied to avoid the packet loss. For real time video traffic, the TCP retransmission may not to applicable due to the delay budget and the data retransmission robustness relies on the RAN retransmission functionality such as RLC and MAC. For URLLC, the retransmission of TCP, PDCP and RLC may not be applicable due to the extremely small delay budget and the robustness of the data transmission mainly relies on the enhancement of the initial transmission and HARQ retransmissions.
URLLC may be used for factory automation. The target packet loss rate is relatively low, for example, 10^-6˜10^-9. ARQ protocol might not be applicable in such cases due to the delay budget limit. The packet loss rate is mainly determined by the residual transmission error in MAC layer. In practice, the residual error of HARQ depends on not only the data decoding error but also the uplink control channel decoding error. For instance, the Negative Acknowledgement (NACK) to Acknowledgement (ACK) error has clear impact on the residual MAC error. The uplink control channel with 0.1% NACK to ACK error rate cannot meet the packet loss rate of the low rate for URLLC traffic.